A. Field of the Invention
This invention relates generally to the field of telecommunications, and more particularly to methods by which telephone calls (either via a personal computer or conventional xe2x80x9cblack telephonexe2x80x9d set) are placed between two parties over a data network such as the Internet.
B. Description of Related Art
Internet telephony presents an attractive alternative for long distance telephone calls, as compared to the public switched telephone network (PSTN), which has been the traditional transmission medium. The primary advantage is cost. Long distance service providers for the PSTN offer domestic services at rates ranging from roughly 10 to 30 cents per minute, and international rates for substantially more, depending on the time of day, day of the week, and the distances involved. In contrast, the cost of Internet telephony anywhere in the world is potentially the cost of a local telephone call to the local Internet telephony provider at one end and the cost of a local call from an Internet telephony service provider at the far end to the destination telephone. Once the call is routed from the local Internet telephony provider onto the Internet, the transmission of the call from the local Internet telephony provider to the far end Internet telephony provider is free to the calling and called party for all practical purposes, regardless of where the two parties are located. The same holds true for whether the call is sent over the Internet or over a private data network.
It is presently contemplated that Internet service providers with a Point of Presence on the Internet will be suitable entities to offer Internet telephony services. The devices that are used by most Internet service providers for Internet access are known as Network Access Servers or Remote Access Servers. These products are commercially available from 3Com Corporation and other telecommunications equipment manufacturers such as Ascend Communications, Lucent Technologies (successor to Livingston Enterprises), and Multitech.
A representative Network Access Server is the Total Control Enterprise Network Hub from 3Com Corporation, described in the patent of Dale M. Walsh, et al., U.S. Pat. No. 5,528,595, which is fully incorporated by reference herein. This device has a telephone line interface that is connected to a high-speed multiplexed digital telephone line connected to the PSTN, such as a T1 line. The device further includes a plurality of digital modems to perform signal conversions on the data from the telephone line channels and a bus network connecting the modems to a network interface card or module. The network interface couples the device to a local or wide area network, such as the Internet service provider backbone network or the Internet. Network Access Servers are particularly suited for use in Internet telephony, as they can be configured with software to perform the functions of a Gateway or Terminal, as defined by the relevant ITU-T H.323 and H.225 specifications. This is particularly so if the device is configured with a general purpose computing platform (such as the EdgeServer card of the Total Control Network Access Server), as described in the pending patent application of William Verthein, Daniel L. Schoo and Todd Landry, U.S. Ser. No. 08/813,173, also incorporated by reference herein.
In a typical Internet telephony scenario, a user will attempt an Internet telephony call with either a touch tone telephone or else a personal computer equipped with a modem, microphone, speaker and a suitable commercially available telephone software package, such as Megaphone(trademark). In either circumstance, the situation may arise in countless types of telephone calls in which the user has to enter alpha-numeric information with their telephone or PC. For example, when the user calls an automated system for processing bank or individual retirement account transactions, the system typically will prompt the user to enter digits on their telephone corresponding to their social security number, password number, and account number. As another example, the pressing of digits on the phone is often necessary in order to navigate through a voice mail menu system, or in order to use an automated flight arrival or departure system maintained by an airline, to name a few others.
When the user presses a number on the telephone key pad, e.g., xe2x80x9c2xe2x80x9d, tone generation circuitry in the telephone (or PC) generates what are known as multifrequency tones. These tones are sometimes referred to as DTMF (Dual Tone MultiFrequency) tones. For example, the number xe2x80x9c2xe2x80x9d is represented by two tones having a frequency of 700 and 1100 Hz. Each digit from 0 to 9 has its own unique set of two tones. Additionally, the # key and the * key also have their own tones associated therewith. These tones are transmitted in-band (i.e., in the voice band) from the user to the far end for processing.
The present inventors have appreciated that the problem can arise in the transmission of telephone calls over a data network where the DTMF tone is subject to noise or distortion before it reaches the modem (or other device or system) coupling the telephone line to the data network. For example, a user may purchase an inexpensive telephone that has a poor quality DTMF tone generator, and use that telephone when dialing over a noisy or poor quality analog PSTN line. Furthermore, when the DTMF tones are transmitted to the modem linking the PSTN to the data network (e.g., the modems in a network access server acting as a Gateway/Terminal), the tones undergo lossy data compression and signal transformation processes before being placed on the data network. As a result, the devices on the network may have difficulty processing the data signals representing the DTMF tones, and in particular discriminating between one set of tones and other. The possibility may arise in which the tones associated with xe2x80x9c1xe2x80x9d are interpreted as the tones associated with xe2x80x9c2xe2x80x9d. Or, the tones may simply not be recognized. Obviously, the occurrence of such an error would, at the very least, be extremely inconvenient to the user, and could be potentially very problematic. For example, mistakes could be made in the handling of the user""s investment account, they could get the wrong flight times from the airlines, they would be unable to leave an important message or leave it with the wrong person, etc.
It is known in the art to detect DTMF tones in a modem, including a modem in a network access server. See the patent to Marc S. Baum, et al., U.S. Pat. No. 5,577,105, assigned to the assignee of the present invention. The Baum et al. ""105 patent is hereby incorporated by reference herein. In the Baum et al. ""105 patent, the DTMF tones are used by the network access server to perform various functions for an incoming call, such as quickly configure a modem to operate in a manner compatible with the transmission requirements of a remotely located modem, call up specific applications programs, or to route a call to a particular destination on a network. However, the potential for downstream mis-identification of the tones or difficulty in processing the tones, and the solution thereof provided by the present invention, is not recognized or appreciated in the above Baum et al. ""105 patent, or other prior art known to the present inventors.
A method is provided for processing a touch tone (e.g., DTMF tones) subject to noise or distortion either during generation of the touch tone or during transmission of the touch tone from the source along a transmission medium, such as an analog telephone line or radio frequency cellular telephone link. In one embodiment, the method comprises the first step of receiving the tone, such as, by receiving the tone in a modem in a network access server or in some other device linking a telephone or time division multiplexed network to a packet switched network. After the tone is received, it is analyzed to determine what digit the tone corresponds to. For example, tone processing circuitry (e.g., a DSP) may determine that the tones are 700 and 1100 Hz, corresponding to the digit xe2x80x9c2xe2x80x9d. The tones are then replaced and substituted with a pure, i.e., substantially noiseless and distortion-free, digital signal corresponding to the tone. Again, this can be performed with suitable digital signal processing circuitry in the modem or other device, such as by a DSP fetching a bit pattern corresponding to the digital signal from a memory, or by use of a digital tone generator circuit. Then, the substantially noiseless and distortion-free, digital signal corresponding to the tone is transmitted onto a data network for receipt by a device on the data network. The data network, such as the Internet, is typically a substantially noise-less transmission medium, so the device on the network receiving the digital replacement tone will have no difficulty detecting the digital signals corresponding to the tone, distinguishing one tone from another, and correlating the tone with the digit pressed at the other end. Thus, the shortcomings in the prior art are avoided.
In one possible embodiment of the invention, the device detecting the touch tones (such as in the modem coupling the PSTN to the data network) may perform additional steps of determining whether the noise or distortion of the tone is below a predetermined threshold level (i.e., the signal to noise ratio is above a predetermined threshold), and if the signal to noise ratio is below the predetermined threshold, responsively replacing the tone with a substantially noiseless and distortion-free digital signal corresponding to the tone. This embodiment would thus pass though relatively noiseless and distortion free tones. However, the simpler and most reliable implementation of the method, and thus the preferred approach, is to replace all detected touch tones with the pure noiseless and distortion-free digital signals corresponding to the detected tones.
In an embodiment of the invention in which the substitution of the tones is performed in a modem, additional processing of the tone may be performed. For example, the substitution of the tone with the replacement pure digital signal may be accompanied by compression and encoding of the substantially noise and distortion free digital signal corresponding to the DTMF tone. Although these will typically be lossy processes, the resulting compressed and encoded tones are still able to be reliably detected and discriminated by devices on the data network.
In another aspect of the invention, an improvement to a network access server is provided. The network access server comprises a telephone line interface coupling the network access server to a PSTN line, a modem, a network interface coupling the network access server to a data network, and a bus complex for connecting the telephone line network, modem and network interface to each other. The modem includes a digital signal processor and either a digital tone generator or a memory storing the bit pattern for digital tones corresponding to the DTMF tones for 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, # and *. The digital signal processor detects a DTMF tone subject to loss or distortion present in a data stream from the PSTN line. Using either the tone generator or memory, the DSP responsively replaces the DTMF tone with a substantially noise-free and distortionless digital signal corresponding to the DTMF tone. The modem transmits the digital replacement signal to the network interface for placement on the data network. Accordingly, the reliability of detection and discrimination of the DTMF tone by a device in the network may be improved.
While one embodiment of the invention is described herein in the context of a network access server coupling a noise-prone PSTN line to a data network, it will be appreciated that the invention can be implemented in other environments where there is an interface between a network carrying tones subject to loss or distortion and a packet-switched network. For example, cellular telephone networks have aggressive voice compression algorithms to optimize bandwidth utilization, and loss-prone radio frequency links, hence the potential for distortion and resulting undetectability of DTMF tones at the far end is pronounced. The method of the invention can be utilized in cellular telephone switches (or conceivably elsewhere in a cellular telephone network) where there is an interface between a time division multiplexed transmission medium carrying cellular phone data and a packet switched network such as the Internet. In one possible embodiment, the invention could be implemented in a modem in a gateway for providing internet access for cellular telephone customers, such as in the modems of the network access server described in the pending patent application of Yingchun Xu, et al., U.S. Ser. No. 08/887,313, assigned to the assignee of the present invention, which is incorporated by reference herein.
These and other features of the preferred and alternative embodiments of the invention will be more apparent from the following detailed description.